Ah, the Writers!
Thanks to Laura for the heads up about Frances Marion, the Wikipedia article about her is interesting. According to the credits at the beginning of the film, the screenplay was adapted from a play by Frank Vosper, which in turn was adapted from a short story by...Agatha Christie! When it comes to source material for a suspense thriller, it is hard to do better than Agatha.
Nothing I can add to what others have already said about the quality and versatility of his acting. I read somewhere that he voiced the usual concern over being type-cast in discussing his role as Sherlock Holmes. He said he wanted to flex his muscles or some such, and try out different things. Unfortunately, he was given absolutely nothing to work with in the other films in which I've seen him. This one is different, and he really shows his stuff.
He is "the Vincent Price Vincent Price could never be" because Mr. Price brought a sense of humour to the horror genre that he never could totally leave behind. In a way, he was his own http://www.google.com/search?hl=en&q=larry+vincent+seymour+horror+host"
rel="nofollow">Elvira/MST3K, subtly inviting the audience to laugh right along with him at the corny melodramatic tropes and stereotypes with which the genre is filled. Basil Rathbone, however, plays his part in this film straight and absolutely to the hilt, with the result that he is really scary!
Sherlock Holmes, as portrayed in the many short stories and four novels by Conan Doyle, is an interesting character, and much has been written about him—from his addiction to stimulation (a good case when he had one, and cocaine when he didn't), to his descent into apathy and depression when he had neither a case nor some coke, to his nearly complete disinterest in people as human beings, to his emotional detachment even when not working. Believe it or not, the same qualities that made Rathbone the unrivaled Holmes until Jeremy Brett came along, also suited him ideally to play this role.
This is the typical (even prototypical, one might say) portrayal of the psychopath as a deranged psychotic killer who is always on the edge of flying apart or breaking down. The argument that "it's only entertainment" unfortunately doesn't work, because this has become the public's conception of what socio/psycho-pathology really is, with the result that real world psychopaths and sociopaths (who are not also psychotic killers) go unrecognized (professionally, they gravitate to law, politics, and corporate culture) until they hurt lots of people—and sometimes, not even then. Writers like Agatha Christie perpetuate the trope because it sells books and movies, but they do a disservice when the public takes this image of the "typical" socio/psychopath for truth.
Part of the portrayal is accurate, though: particularly, with a charismatic (paranoid) personality (they make great cult leaders) and glib persuasiveness, socio/psychopaths ooze a (false) charm that is quite disarming, in the same way that Ann Harding's character is disarmed. Amoral and egocentric, they have a complete disregard for anyone but themselves, and can tell the most outrageous lies imaginable with complete calmness and sincerity. Their chief advantage is stealth: few people can believe that anyone could be so devoid of the qualities that make social life possible, even when confronted with such a person. By the time that most people realize they have been taken in by one of these, the harm is already done. That's really scary, so it makes great material (with a little seasoning added, like the psychotic killer stuff) for suspense thrillers like this one.
For anyone who is interested in this topic, it may be worthwhile to compare this film to another suspense thriller, http://www.archive.org/details/Gaslight_1940" rel="nofollow">Gaslight,
the title of which has entered our lexicon as "http://en.wikipedia.org/wiki/Gaslighting"
rel="nofollow">gaslighting," and also to http://www.archive.org/details/big_trees" rel="nofollow">The Big Trees,
in which Kirk Douglas plays the role of a greedy land swindler. Both leave out the psychotic killer stuff, and portray the glib and charismatic aspects of the socio/psychopath faithfully.
...is very poor with blurry picture and mushy sound, but it is still possible to see and hear the action. Filtering the sound through an equalizer with the treble boosted and the bass turned down makes it easier to understand the dialogue, even though doing that also amplifies hiss and other noises.